Validation of OCM-3 chlorophyll-a in the Eastern Arabian Sea using in situ measurements: a comparative analysis with MODIS-Aqua, VIIRS, and Sentinel-3B
摘要
Chlorophyll-a (Chl-a) is a primary photosynthetic pigment of oceanic phytoplankton, which plays a crucial role in determining the colour of seawater. Consequently, estimation of Chl-a is a key indicator for monitoring phytoplankton biomass and marine ecosystem health. In this context, the present study evaluates the performance of the newly launched Indian satellite Ocean Colour Monitor-3 (OCM-3) sensor onboard EOS-06/Oceansat-3, in estimating surface Chl-a concentrations, in comparison with other global ocean colour sensors, MODIS-Aqua, VIIRS, and Sentinel-3B, in the Eastern Arabian Sea (EAS) during the late-winter monsoon. In situ Chl-a data collected from 63 stations during a ship cruise onboard CRV Sagar Tara were compared with satellite-derived Chl-a. Statistical metrics such as RMSE, bias, slope, R2, and uncertainty indices (MAPD, MRPD, UPD) were employed to assess sensor accuracy. Chl-a concentrations varied across the EAS, higher in the North Eastern Arabian Sea (NEAS) (2.60 ±1.06 mg/m3) compared to the South Eastern Arabian Sea (SEAS) (0.78 ± 0.50 mg/m3). The high Chl-a concentration in the NEAS is attributed to the impact of convective mixing and nutrient enrichment. From this study, the performance of sensor assessment shows that MODIS-Aqua has a higher correlation (r = 0.71) and accuracy in Chl-a distribution followed by OCM-3, VIIRS, and Sentinel-3B along the EAS. In particular, the OCM-3 showed performance comparable to MODIS-Aqua in mesotrophic (0.5–1 mg/m3) Chl-a distribution in the EAS, characterized by a low RMSE (0.432), although with a relatively higher bias compared to MODIS-Aqua. The OCM-3 showed a systematic overestimation of low Chl-a (<0.5 mg/m3) and a consistent underestimation of high Chl-a (>1 mg/m3), particularly in optically complex coastal waters influenced by total suspended matter (TSM) and coloured dissolved organic matter (CDOM). This study revealed that the OCM-3 Chl-a need to be evaluated further with an extensive dataset around the EAS to improve the regional algorithms for accurate measurements of Chl-a.
Research highlightsChl-a concentration was higher in the North Eastern Arabian Sea (NEAS) than the South Eastern Arabian Sea (SEAS) during the late-winter monsoon. The high Chl-a observed in the NEAS was primarily driven by convective mixing. OCM-3 sensor showed performance comparable to MODIS-Aqua in mesotrophic waters (0.5–1 mg/m3), with low RMSE but relatively higher bias. The OCM-3 showed a systematic overestimation of low Chl-a (<0.5 mg/m3) and a consistent underestimation of high Chl-a (>1 mg/m3).